Why will my program not flag characters? - c

Question

I'm making this program using switch statements that will assign letter grades based on if the user enters numbers 0 - 10. If the user enters a number that is not 0-10, the program outputs an error message and has the user re-enter. However, if the user enters a character the program will loop at the default case. I want it to output the error message from the default case once, and have them re-enter if they enter a character. I'm not sure as to why it loops the default case when a character is entered though.
#include <stdio.h>
int main()
{
int grade;
int r;
while((r = scanf("%i", &grade)) != EOF)
{
switch(grade)
{
case 10:
case 9:
printf("Your grade is an A\n");
break;
case 8:
printf("Your grade is a B\n");
break;
case 7:
printf("Your grade is a C\n");
break;
case 6:
printf("Your grade is a D\n");
break;
case 5:
case 4:
case 3:
case 2:
case 1:
case 0:
printf("Your grade is an F\n");
break;
default:
printf("Invalid score, please re-enter\n");
}
}
return 0;
}

Answer 1

Try something like:
#include <stdio.h>
int main()
{
int grade;
int r=0;
while(r != 1)
{
scanf("%i", &grade);
switch(grade)
{
case 10:
case 9:
printf("Your grade is an A\n");
r=1
break;
case 8:
printf("Your grade is a B\n");
r=1
break;
case 7:
printf("Your grade is a C\n");
r=1
break;
case 6:
printf("Your grade is a D\n");
r=1
break;
case 5:
case 4:
case 3:
case 2:
case 1:
case 0:
printf("Your grade is an F\n");
r=1
break;
default:
printf("Invalid score, please re-enter\n");
break;
}
}
return 0;
}

Answer 2

This will clear the input buffer on an invalid input and allow a retry.
#include <stdio.h>
int main()
{
int grade;
int r;
while((r = scanf("%i", &grade)) != EOF)
{
if ( r != 1) {//r == 1 is successful input of integer
grade = -1;//reset grade on invalid input
}
switch(grade)
{
case 10:
case 9:
printf("Your grade is an A\n");
break;
case 8:
printf("Your grade is a B\n");
break;
case 7:
printf("Your grade is a C\n");
break;
case 6:
printf("Your grade is a D\n");
break;
case 5:
case 4:
case 3:
case 2:
case 1:
case 0:
printf("Your grade is an F\n");
break;
default:
printf("Invalid score, please re-enter\n");
while ( getchar() != '\n');//clear input buffer
}
}
return 0;
}

Answer 3

The reason your code always loops is because there is no way to exit out of your while other than to kill the program. Remember that break only breaks out of the inner-most switch or loop.
The cleanest way to break out of multiple levels is to use a flag. One way to do what you want is like this:
bool valid_grade = false;
while(!valid_grade && (r = scanf("%i", &grade)) != EOF)
{
valid_grade = true;
switch(grade)
{
case 10:
// unchanged from your code
default:
valid_grade = false;
printf("Invalid score, please re-enter\n");
}
}